I've noticed in the newer Retropie Builds of Attract mode that some of my transparent .pngs now have borders. I double checked the same .pngs on my Windows build and they dont appear. I also checked to see if there was infact any white or colored pixels around the border of the images in question. They dont exist but for some reason I'm getting this border artifact in Attract Mode now on my Raspberry Pi.
Further testing reveals that lower resolution wheel images are more likely to get this artifact. So perhaps its related to how Attract Mode handles image scaling.